Field of the Invention
The present invention relates to a method and apparatus for measuring the temperature of a semiconductor substrate, particularly in-situ during semiconductor processing.
Description of Related Art
Production of semiconductor devices, displays, photovoltaics, etc., proceeds in a sequence of steps, each step having parameters optimized for maximum device yield. Among the controlled parameters strongly affecting yield is the temperature of the substrate from which devices are formed, because temperature strongly affects the rate of and outcome of a processing step. While ensuring that the temperature of the substrate is within limits for each processing step, it is also equally important to maintain temperature steady over time, i.e. from substrate to substrate, and substrate lot to substrate lot, to prevent process drift. It is also very important to maintain uniformity of temperature across the substrate during each processing step, such that properties of devices do not vary considerably from one region of the substrate to another.
The goal of maintaining control of substrate temperature, and its uniformity across the substrate and over multiple substrates requires monitoring of substrate temperature during processing, preferably across multiple locations on the substrate. Active monitoring of substrate temperature is frequently complicated by the fact that processing occurs in harsh and unfavorable environments. For example, in situ temperature measurement devices need to be unaffected by the aggressive chemistries and environments (e.g. plasma) sometimes used in semiconductor processing. In plasma processing environments, strong RF coupling from the RF excitation method used to drive a plasma in the plasma processing system can lead to noisy and erroneous temperature measurements due to induced currents in unshielded or poorly-shielded temperature sensor circuits. Some temperature measurement methods have sought to solve these issues by placing temperature sensors inside the substrate support, but such a measurement is further complicated by the fact that the substrate is seldom in good thermal contact with the substrate support, so the reading of the temperature sensors embedded within the substrate support is rarely accurate due to the temperature difference (i.e. “jump”) between the substrate and the substrate support. Attempts to directly measure the substrate temperature have typically involved some sort of single or multi-point optical temperature measurement system installed inside the processing chamber, but such system also have their shortcomings, such as the tendency of optical components to get coated with processing byproducts adhering to the wall of the processing chamber, thus affecting measurement accuracy; and also high cost.
Therefore, there still exists a need for a robust and inexpensive system and associated method for measuring a temperature and temperature distribution of a substrate itself, during processing. Direct ultrasonic measurement of substrate temperature, particularly as described hereinafter, addresses most of the aforementioned concerns.